An MOS transistor, such as a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) or an IGBT (Insulated Gate Bipolar Transistor), is a voltage controlled device that includes an input capacitance. The MOS transistor can be switched on and off by controlling the voltage across the input capacitance. This input capacitance is formed by a gate electrode, a gate dielectric, and doped semiconductor regions that adjoins the gate dielectric, and includes a gate-source capacitance (in case of a MOSFET), or gate-emitter capacitance (in case of an IGBT). A normally-off MOS transistor switches on when a drive voltage across the gate-source capacitance reaches a threshold voltage, and switches off when the drive voltage falls below the threshold voltage.
In many applications it is desirable to switch on the MOS transistor as fast as possible, but to prevent steep switching edges. “Switching edges” are edges of a load current through the MOS transistor, and of a voltage across a load path (the drain-source path in a MOSFET, and the collector-emitter path in an IGBT) of the MOS transistor in the switching process.
According to a known concept of switching on an MOS transistor, the input capacitance is charged with a relatively high charging current in a precharging phase until the voltage across the input capacitance reaches the threshold voltage, and is charged with a lower current afterwards. In the precharging phase, the MOS transistor is in an off-state, so that a high precharging current helps to obtain a high switching speed but does not result in steep switching edges. The lower charging current after the precharging phase helps to prevent steep switching edges and further charges the input capacitance until the voltage across the input capacitance reaches a voltage level where the MOS transistor has a minimum on-resistance.
However, this concept requires to detect when the voltage across the input capacitance reaches the threshold voltage. This may include detecting at least one of the drive voltage across the input capacitance, detecting the load current (that starts to increase as the drive voltage reaches the threshold voltage), and the load-path voltage (that starts to decrease as the drive voltage reaches the threshold voltage). However, parasitic effects, such as voltages across line inductances, may make it difficult to precisely measure these parameters. Further, propagation delays may cause the input capacitance to be charged with the high precharging current between the time of detecting the threshold voltage and the time when the charging current is reduced to the lower charging current.
There is therefore a need for an improved method for switching an MOS transistor.